The CO2 permeation experiments were performed on a microporous carbon molecular sieve membrane (CMSM) at 25 degreesC and over a wide pressure range. The permeation time lag of the penetrant was employed to study the concentration dependence of the diffusion coefficient. The analytical solutions are derived for permeation systems with a nonlinear sorption isotherm (Langmuir) and a concentration-dependent diffusivity [with the functional form of D-mu(c) D-mu(0) = 1/(1 - theta)(n), where theta is the ratio of surface coverage and n greater than or equal to 1]. By comparison of the experimental data with the theoretical models, it is found that, in this CMSM, the apparent diffusivity of CO2 takes a concentration dependence stronger than that described by the traditional HIO model [i.e., D-mu(c)/D-mu(0) = 1/(1 - theta)].